1. Field of the Invention
The present invention relates to a valve drive mechanism for an engine which is variable in valve lift and has a valve lifter or tappet which selectively transmits rotation of different lift cams.
2. Description of Related Art
There has been known various valve drive mechanisms which can drive valves with variable valve lifts. For example in Japanese Unexamined Patent Publication No. 3-46642 a valve drive mechanism in which a valve lift is variable. This valve drive mechanism for an engine equipped with twin intake valves and twin exhaust valves per cylinder has a tappet assembly which comprises a center tappet and a pair of side tappets arranged such that the center tappet is interposed between the side tappets. The tappet assembly further comprises locking/unlocking pins operative to lock or couple the center tappet to the side tappets, respectively, so as to force the center tappet and the side tappets to move up and down as one whole or unlock or uncouple the center tappet from the side tappets so as to allow the center tappet to move up and down relatively to the side tappets, thereby selectively transmitting rotation of a high lift center cam and low lift side cams to the twin valves, in other words, driving the twin valves with a variable valve lift.
The prior art valve drive mechanism couples and uncouples these center and side tappets in locking/unlocking positions each of which is on a line passing vertical center axes of the side tappets but offset toward a vertical center axis of the center tappet from the vertical center axes of the side tappets, respectively, in an axial direction of a camshaft. This possibly causes an inclination of each side tappet toward the center tappet while the center tappet and the side tappet coupled together is driven as one whole by the high lift center cam, which is one of causes of undesirable wear on the side tappet and a guide tappet guide. In addition, the tappet assembly is apt to cause a relative inclination between the center tappet and the side tappet because it employs a single locking/unlocking pin in order to couple center tappet to each side tappet. This is another one of causes of undesirable wear on the side tappet and the tappet guide. The center tappet has a circular tappet head which provides only a short length of slide contact with the center cam, so that the center tappet is subjected to a great pressure on the tappet head. This results in wear on the tappet head of the center tappet.
It is therefore an object of the present invention to provide a valve drive mechanism which provides a center tappet head with a long length of slide contact with a center cam.
It is another object of the present invention to provide a valve drive mechanism which prevents an inclination of side tappets due to integral up and down movement while the center and side tappets are operated as one whole by a high lift cam so as thereby to prevent undesirable wear on the tappets and a tappet guide.
The above objects of the present invention are accomplished by a valve drive mechanism for an engine having twin intake valves and twin exhaust valves per cylinder for driving each twin valves with variable valve lift by one center cam, preferably a high lift cam, on a camshaft, namely an intake camshaft or an exhaust camshaft, and a pair of side cams, preferably low lift cams, on the camshaft that are arranged on opposite sides of the center cam in an axial direction of the camshaft and different in cam profile from the center cam. The valve drive mechanism comprises one center tappet operative to transmit rotation of the high center cam as reciprocating movement to said valve, a pair of side tappets, between which the center tappet is interposed so as to be movable relatively to the side tappets in a direction of valve lift by a tappet spring and the center cam alternatively, operative to transmit rotation of the low lift side comes as reciprocating movement to the valves through valve stems, respectively, and a locking/unlocking mechanism operative to mechanically couple and uncouple the center tappet and the side tappets in locking/unlocking positions each of which is in a vertical plane including a substantial vertical center axis of each of the side tappets and intersecting perpendicularly to the axial direction of the camshaft so as thereby to selectively transmit the rotation of the high lift center cam and the low lift side cams as reciprocal movement to the valves, respectively. The center tappet preferably has a generally rectangular tappet head. In this connection, the locking/unlocking mechanism is disposed on each of opposite sides of a line passing the substantial vertical center axes of the side tappets in a rotational direction of camshaft and offset from the line in a rotational direction of the cams.
Each side tappet preferably has a generally cylindrical hollow shell formed with a rectangular opening. These side tappets are disposed side by side at a specified distance from each other in the axial direction of camshaft such that the openings face each other in the axial direction of camshaft. The center tappet is disposed between the side tappets such as to be received in the rectangular openings.
The valve drive mechanism may further comprise a pair of guide rods disposed in the side tappets, respectively, by which the center tappet is supported for slide movement. These guide rods are preferably coaxial with the vertical center axes of the side tappets, respectively.
The locking/unlocking mechanism comprises a locking/unlocking pin disposed in the center tappet, a hydraulically operated locking plunger disposed in one of the side tappets so as to abuts against one end of the locking/unlocking pin, and a spring loaded unlocking receiver disposed in another one of the side tappets so as to abut against another end of the locking/unlocking pin. Specifically, the locking/unlocking pin is movably received in a center guide bore which is formed in the center tappet so as to extend along a line offset from the line passing the substantial vertical center axes of the side tappets in the rotational direction of camshaft and parallel to the axial direction of camshaft between the vertical planes. The hydraulically operated locking plunger is movably received in a first guide bore which is formed in alignment with the center guide bore in the one side tappet. The spring loaded unlocking receiver is movably received in a second guide bore which is formed in alignment with the center guide bore in the other side tappet. The locking/unlocking pin is forced to partly enter the second guide bore by the hydraulically operated locking plunger when the hydraulically operated locking plunger is forced by hydraulic oil to partly enter the center guide bore, so as to mechanically couple the center tappet to the side tappets. On the other hand, the locking/unlocking pin is forced by the spring loaded unlocking receiver to come out of the second guide bore and to return the hydraulically operated locking plunger into the first guide bore when the hydraulically operated locking plunger is released from the hydraulic oil.
The locking/unlocking mechanism further comprises a hydraulic oil path arrangement for introducing hydraulic oil from an oil gallery in the cylinder head to the first guide bore so as to apply hydraulic pressure on the hydraulically operated locking plunger.
Specifically, the hydraulic oil path arrangement comprises an oil channel formed in the one side tappet through which the hydraulic oil is introduced into the first guide bore, more preferably into an oil chamber that may be formed at one of opposite ends of the first guide bore in the one side tappet and extend coaxially with the first guide bore from the first guide bore to an outer wall of the one side tappet. The hydraulic oil path arrangement may further comprises an oil channel formed in the one side tappet through which hydraulic oil is introduced into the oil chamber. The oil channel may be communicate with an oil gallery formed in parallel to camshaft in the cylinder head by a branch oil channel that is formed in the tappet guide. The branch oil channel partly opens to a tappet guide bore of the tappet guide in which the one side tappet is received so as to communicate the tappet guide bore with the branch oil passage.
The hydraulic oil path arrangement may further comprise a vertical oil channel formed in the one side tappet so as to communicate both the oil channels of the locking/unlocking mechanisms, The vertical oil channel preferably has a length in the direction of valve lift sufficient to keep communication of both oil channels of the locking/unlocking mechanisms with the branch oil channel while the tappet assembly reciprocally moves in the direction of valve lift.
According to the valve drive mechanism equipped with the tappet assembly in which locking/unlocking is performed in a position which is in a vertical plane including a substantial center axis of the side tappet and intersecting perpendicularly to the axial direction of camshaft, an inclination of the side tappets is prevented or significantly reduced during integral up and down movement of the center tappet and the side tappets operated as one whole by the high lift cam. Further, according to the valve drive mechanism equipped with the tappet assembly in which the center tappet is supported for slide movement by the pair of guide rods disposed preferably coaxially with the vertical center axes of the side tappets, respectively, an inclination of the side tappets with respect to the center tappet is prevented or significantly reduced during movement of the center tappet relative to the side tappets while the center tappet is driven independently from the side tappets by the high lift cam. As a result, the side tappets and the guide tappet guide are prevented from undesirable wear.
The configuration of the center tappet head that is generally rectangular provides the center tappet with a long length of slide contact with the center cam and, in addition, enables disposing the locking/unlocking mechanism on each of opposite sides of the axial direction of camshaft. The locking/unlocking mechanisms are offset from the line passing the vertical center axes of the side tappets in the rotational direction of cams and disposed on opposite sides of the axial direction of camshaft, so that the tappets are prevented from inclining in the rotational direction of cams. This guarantees the tappets to move up and down precisely in the direction of valve lift.
The locking/unlocking mechanism comprises the locking/unlocking pin moveably received in the center guide bore of the center tappet, the hydraulically operated locking plunger slidably received in the first guide bore of the one side tappets so as to abuts against one of the opposite ends of the locking/unlocking pin, and the spring loaded unlocking receiver slidably received in the second bore of the other side tappets so as to abut against another end of the locking/unlocking pin. The first guide bore is formed with an oil chamber into which hydraulic oil introduced so as to force the hydraulically operated plunger in the first guide bore. The locking/unlocking mechanism operates such that the locking/unlocking pin is forced to partly enter the second guide bore by the hydraulically operated locking plunger when the hydraulically operated locking plunger is forced by hydraulic oil to partly enter the center guide bore, so as to mechanically couple the center tappet to the side tappets and, on the other hand, the locking/unlocking pin is forced by the spring loaded unlocking receiver to come out of the second guide bore and to return the hydraulically operated locking plunger into the first guide bore when the hydraulically operated locking plunger is released from the hydraulic oil.
According to the valve drive mechanism equipped with the locking/unlocking mechanism thus structured, the locking/unlocking pin is reliably actuated for locking operation by hydraulic oil that an oil pump provides at a high pressure while the engine operates at a high speed.
The center tappet having a generally rectangular tappet head is disposed between the rectangular openings formed in the generally cylindrically shaped shells of the side tappets. This configuration of the tappet assembly can provides the center tappet with a long length of slide contact with the center cam having a high lift for high speed operation. Further, each low lift side cam that is more frequently used than the high lift center cam rides on the side tappet at a position in alignment with the valve stem. This prevents or significantly reduces an inclination of the tappets due to an offset of the valve stem from the contact point of the low rift side cam with the tappet.